The present invention relates to semiconductor integrated circuits and more particularly to an integrated circuit having protection against a reverse connected power supply.
Integrated circuits (IC's) are typically designed to perform digital or analogue circuit functions. Their small size makes them especially suitable for incorporation in portable equipment, which is typically powered by batteries. In these battery powered applications, the danger exists that when replacing a used battery with a fresh one, the new battery may be connected backward from normal. Most semiconductor integrated circuits are so constructed that a polarity reversal of the d.c. power source will cause permanent and catastrophic damage to the circuit components. It is desirable that low cost protective means against an inadvertently reversed battery, be provided within the integrated circuit itself. A known means for accomplishing this, comprises forming by standard IC methods a normal p-n junction diode in the circuit substrate and connecting this diode in series relationship with the d.c. power supply (battery) and d.c. power buss terminals of the primary portion of the IC. The diode is polarized so that it is "forward biased" when the battery is properly connected and vice versa.
Unfortunately the simplest and most straightforward means for fabricating the diode often results in the inadvertent formation of a parasitic bipolar transistor or an SCR that degrades the performance of the primary circuit. Such is likely to be the case, for example, when the diode is formed in a portion of the isolated pocket containing a collector resistor. The hazard also exists when forming the protective diode in a portion of the collector region of a transistor. Furthermore, even if through good fortune or special design the formation of a parasitic transistor is avoided, the forward drop (V.sub.BE) of the p-n diode in normal operation is about 0.6 volts for silicon and the effective d.c. supply voltage, that is available to the primary portion of the I.C., is diminished by 0.6 volts. This loss may be intolerable where the circuit is intended to use a low voltage supply, for example, as provided by two or even three series connected dry cells of 1.5 volts each.
It is therefore an object of this invention to provide within an I.C. a reverse battery protection means, without creating parasitic transistors.
It is a further object of this invention to provide within an I.C. a reverse battery protection means, with a minimum reduction in the effective d.c. power supply voltage.
It is yet a further object of this invention to provide a simple low cost reverse battery protective means in an I.C.